Oil wells and the like are commonly drilled with rock bits having rotary cones with cemented tungsten carbide inserts. As such a bit is rotated on the bottom of a drill string in a well, the cones rotate and the carbide inserts bear against the rock formation, crushing and chipping the rock for extending the depth of the hole. Typical inserts have a cylindrical body which is pressed into a hole in such a cone and a somewhat blunt converging end that protrudes from the face of the cone. The converging end of the insert may be generally conical, roughly hemispherical, or have a somewhat chisel-like shape. Another type of bit for drilling rock employs a steel body in which similar tungsten carbide inserts are embedded. The bit is hammered against the bottom of the hole for shattering rock and gradually rotated as it drills. Inserts provided in practice of this invention may be used in either type of rock bit, or in other related devices such as underreamers.
Since the tungsten carbide inserts are the parts of the rock bit that engage and drill the rock, it is important to minimize wear and breakage of such inserts. Tungsten carbide inserts for rock bits are made by sintering a mixture of tungsten carbide (WC) powder and cobalt to form a dense body with very little porosity. Two important properties of such inserts are wear resistance and toughness. It is desirable to enhance the hardness of an insert adjacent to its surface where it engages the rock formation and maintain toughness for minimizing breakage of the insert as it is used.
In rock bits designed for a particular type of service, one needs to have an appropriate balance between hardness and toughness. Hard inserts resist wear during drilling. On the other hand, a hard insert may be susceptible to fracture under the impact loads and other abuses necessarily involved in drilling wells. Enhanced toughness is also advantageous, since the part of the insert extending beyond the face of the cone does not need to be as blunt to resist fracture. This means that a longer, more aggressive cutting structure can be employed on a rock bit where fracture toughness is adequate. Thus, soft formation bits may have longer insert protrusion than bits intended for use on harder rock formations.
In essentially all bits, it is desirable to have high hardness and wear resistance and relatively large insert protrusion. Achievement of these desiderata may, however, be limited by a lack of fracture toughness in the main body of the insert. It is desirable to have a hard surface and a tough body. Of course, hardness and toughness throughout the insert is also desirable.
Composite rock bit inserts have been made comprising a layer of polycrystalline diamond on the protruding, converging end of a cemented tungsten carbide insert. This provides a high hardness at the surface and a tough body within the insert. There are appreciable differences in the mechanical and thermal expansion properties of such a polycrystalline diamond layer and the underlying cemented tungsten carbide. A transition layer, comprising a mixture of carbide and diamond crystals, has, therefore, been provided between the polycrystalline diamond layer and the principal body of the carbide insert. Such an insert is shown, for example, in U.S. Pat. No. 4,694,918.
Such an insert may be made by forming a layer of diamond crystals mixed with a small amount of cobalt. Over this there is formed a layer containing a mixture of diamond crystals and precemented tungsten carbide particles. One or more additional layers containing a different proportion of carbide and diamond may be added. A cemented tungsten carbide blank is then placed on the final layer having a mixture of carbide and diamond. This entire assembly is then placed in a very high pressure press and subjected to sufficiently high pressure and elevated temperature to be in a region where diamond is thermodynamically stable. Exemplary minimum temperature is about 1200.degree. C. and an exemplary minimum pressure is about 40 to 45 kilobars.
The assembly is heated and cooled under elevated pressure. This results in formation of a layer of polycrystalline diamond tightly adherent to the cemented tungsten carbide, with one or more transition layers between the polycrystalline diamond and the cemented tungsten carbide body. Such transition layers are a mixture of diamond crystals and precemented tungsten carbide. The polycrystalline diamond layer has high hardness. The cemented tungsten carbide main body of the insert has good toughness. The transition layer or layers help accommodate the differences in thermal expansion and mechanical properties between the polycrystalline diamond and the cemented tungsten carbide.
It is desirable to provide other techniques for forming a transition layer for a rock bit insert having a polycrystalline diamond surface. It is also desirable to provide a rock bit insert having diamond particles distributed in a matrix of tungsten carbide and cobalt. It is also desirable to enhance the wear resistance of a cemented tungsten carbide rock bit insert without significantly degrading toughness.